This invention relates to silicon-based microfabricated motors, and particularly to motors for miniature robots.
Several groups are working on integrated circuit motors or actuators. There have been some early successes, but hurdles still remain. Most of the existing microfabricated structures were developed to produce microsensors. Pressure sensors, chemical sensors and accelerometers are just a few of the wide variety of microsensors now commercially available which convert other variables to electrical signals in order to transduce some physical phenomenon into information. Due to the fact that these sensors can be both mass produced and fabricated with on-chip signal processing, they are rapidly finding homes in such places as the automobile and appliance industries.
The basic technology which makes it possible to fabricate freely moving components on chips is termed silicon micro-machining. Micro-machining processes etch structural shapes in silicon for mechanical purposes. Mechanical structures such as cantilever beams and bridges have been etched in silicon in sizes on the order of a few tens of microns.
Known microfabricated motors rely on variable capacitance forces to create motion. A typical microfabricated motor is the variable capacitance micromotor, which uses capacitive forces to cause a rotor to slide around a bearing. In this motor, friction between the bearing and rotor creates losses.
Other, large scale motors have been designed using piezoelectric phenomena for mechanical force. For example, macroscopic travelling wave motors made out of bulk ceramic PZT have been designed with very high efficiency, and some even appear commercially (see Inaba et al., "Piezoelectric Ultrasonic Motor", Proceedings of the 1987 IEEE Ultrasonics Symposium, pp. 747-756, and Kumada, "A Piezoelectric Ultrasonic Motor", Japanese Journal of Applied Physics, Vol. 24 (1985) Supplement 24-2, pp. 739-741).
Travelling wave motors also have inherent gear reduction due to the rectification of high frequency vibratory motion into continuous unidirectional motion. The resulting high power, low mass, low speed motors are ideal for robotic applications as complex gearing can be omitted.
Of the thirty-two different crystal classes, twenty-one have lattice formations with an inherent asymmetry. Twenty of those twenty-one crystals exhibit piezoelectric properties, which means that application of a voltage across the material causes a mechanical deformation and, conversely, stressing the material produces an electrical signal.
Of the twenty substances that have piezoelectric attributes, ten contain an electric dipole moment in the unstrained condition, which leads to pyroelectric characteristics. A pyroelectric material creates an electrical signal when the crystal is exposed to a change in temperature.
Some of the ten pyroelectric materials also display ferroelectric traits. A ferroelectric material can have its polarization dipole reoriented in direction through the application of a strong electric field. After the electric field is removed, the crystal retains the polarization direction, effectively acting as a solid-state switch.
Ferroelectrics, then, being a subset of pyroelectrics and piezoelectrics, contain attributes of all three. In addition, ferroelectric materials are characterized by having very high dielectric constants.
Piezoelectric, pyroelectric, and ferroelectric phenomena have been used in a wide variety of applications. Materials that are predominantly piezoelectric are often used in items such as speakers, touch sensors or microphones and can be found in bulk ceramic or thin film form. Ceramics with large pyroelectric coefficients are used in applications which sense changes in infrared energy such as burglar alarms or night-vision scopes. Recently, some materials which exhibit ferroelectric properties have been produced in thin film form and incorporated into memory chips to create non-volatile random access memories.
Known robots are designed to individually complete such tasks as window washing, automated manufacturing, or other physical duties which are too hazardous, difficult, or mundane for humans to perform. It has been found that the physical strength of robots can easily exceed that of human beings, but the programming of such robots to perform all but the most repetitive tasks has proven difficult.
Known robot designs are quite expensive, owing mostly to the expense of suitable mechanical actuators and the attendant power systems.